1. Field of the Invention
The present invention relates to a twin-roll continuous casting device which includes a shroud for inerting the casting space defined between the said rolls.
2. Description of the Prior Art
Twin-roll continuous casting plants are already known, especially for direct production of thin steel strip, which include two counterrotating rolls, having parallel axes, defining between them a casting space into which is poured molten metal which solidifies on contact with the cooled walls of the rolls and is extracted in the form of strip while the said rolls are rotating.
A known problem is that the metal contained in the casting space tends to cool at the surface and thus to create parasitic solidifications which impair correct operation of the plant. In order to remedy this problem, it has already been proposed to place a thermally insulating hood above the casting space.
Moreover, in order to prevent the molten metal from oxidizing and from starting to solidify, it has also been proposed to use a shroud system into which an inerting gas is injected in order to prevent ambient air from coming into contact with the liquid metal and the skin of metal which is solidifying. In order to prevent an excessive consumption of this inerting gas, the hood is placed as close as possible to the surface of the rolls, without touching them however, as well as to the upper part of the side walls. Since the rolls are rotating and, in addition, may deform by expansion, it is not possible to ensure good sealing between the said hood and the rolls using static seals. In addition, such seals would run the risk of damaging the surface of the rolls. Thus, it has already been proposed to ensure sealing by blowing an inert gas into the zone where the rolls are covered by the hood, in order both to prevent air from getting in under the hood and to prevent the inerting gas from leaking away outside the protected volume of the mould.
Thus, document EP-A-0,409,645 describes a plant in which channels for injecting an inert gas are arranged against the longitudinal side walls of a hood which covers the casting space, these channels including a slot facing the surface of the rolls so as to blow the said inert gas towards this surface, in order to remove therefrom the film of air entrained by the rolls while they are rotating, and particularly the air contained in the hollows forming the rugosity of the said surface, this being done so as to prevent the said air from penetrating the casting space and therefore to maintain a non-oxidizing atmosphere therein.
In order to distribute the inert gas over the entire width of the rolls, provision is made for the said channels to be able to be partitioned. The document mentioned hereinabove also describes an alternative embodiment in which an external cover is fixed to the lower edge of the side wall of the hood, and positioned adjacent to the surface of the rolls, and includes at its outer edge a channel for influx of inert gas, this being arranged so as to prevent, even more effectively, air from entering the casting space because of the fact that the inside of the said cover is filled with inert gas.
However, these systems are not completely satisfactory since, although they seem a priori capable of preventing air from entering the casting space, they do not enable the gas supplied into this space to be effectively controlled because of the fact that the inert gas blown in via the slots in the channels can be distributed both upstream and downstream of these slots. Since the amount of this gas discharged upstream, that is to say towards the outside, can vary depending especially on the space between the said channels and the surface of the corresponding roll, and on the tangential speed of the latter or on the amount of air drawn in by the rotation of the rolls, it follows that the flow of the remaining gas directed downstream, that is to say towards the casting space, is also variable. Such variations are deleterious, both for the manufacturing process and for the cast product, since an excess of gas may lead to the formation of parasitic solidifications, due to the cooling effect which it exerts on the cast metal, or to the said metal being saturated with gas, or else to a variation in the profile of the roll.